Hitherto, it has been demanded to transmit video and audio data with least delay in, for example, communication systems using video and audio, such as bidirectional video conference systems, and in mixed systems with a non-compression environment in broadcasting stations or the like. In recent years in particular, the amount of data has been increasing as the quality of video and audio has been improved, and accordingly, image compression at the time of transmission has become essential. It has been demanded to suppress an increase in delay time due to the image compression and to perform data transmission with lower delay.
For example, in MPEG (Moving Picture Experts Group) and H.26x compression systems, the compression ratio has been increased based on motion prediction. When motion prediction is performed, the algorithm becomes complicated, and the processing time involved increases in proportion to the square of the frame size. In principle, an encoding delay of a few frames occurs. When bidirectional real-time communication is to be performed, the delay time becomes almost an allowable delay time of 250 ms, which is a length that is not negligible.
Also, intraframe codecs represented by JPEG (Joint Photographic Experts Group) 2000 do not use interframe difference information. Thus, delay such as that described above does not occur. However, since compression is done on a frame-by-frame basis, the codec must wait at least one frame before the start of encoding. Since current general systems employ 30 frames per second in many cases, a waiting time of about 16 ms is necessary before the start of encoding.
In addition, as the quality of video and audio has become improved, it has also been demanded to improve the quality of image compression. JPEG 2000 adopts a compression coding scheme combining high-efficiency entropy coding using bit modeling in increments of a bit plane and arithmetic coding with wavelet transform. For example, a wavelet transform method that has further improved the encoding efficiency has been described in Patent Document 1.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-130800